A peristaltic pump is one in which pumping is accomplished by pinching or squeezing the walls of a tube which contains material to be pumped such that the tube is collapsed shut at successive points along its length. In some forms, the pump tube is resilient, and in relaxed condition is maintained substantially cylindrical in cross-section by its renitence. When such a tube is used in the most common pump form, the form in which a cam presses the tube against a cylindrical inner wall, the tube becomes distorted. It tends to be stretched and twisted with each cam pass. The effect of that is two-fold. The stretching reduces tube life, and the twisting causes the tubing to change its position in the pump housing, complicating housing design and increasing opportunity for malfunction.
Most peristaltic pumps employ more than one cam, usually in the form of rollers, which rotate around a fixed axis. The pump tube is flexible and usually is resilient. Its ends are fixed relative to the axis of cam rotation. The mid-region of the tube is extended around the cam set in the plane of cam set rotation. In some cases the tube is not forced against the wall of the pump cavity. Instead, the tube is stretched so tightly that it is pinched closed at the point engaged by the cam. In the region between the points of cam engagement, the tube lumen is open to accommodate fluid. In other cases the tube is housed in a circular, usually cylindrical, cavity, and the tube is pinched shut at the point of engagement by the cam because the cam presses the tube against the circular cavity wall. In either case the tube is formed into a partial loop, into hairpin or horseshoe shape. The inner circumference is less than the outer circumference. If the tubing was formed as a straight length it must twist along its length when bent to horseshoe shape to compensate for the difference in inner and outer circumference. When the resilient tubing is squeezed closed the ratio of circumferences is changed and the renitence of the tube wall urges a reduction in the degree of twist. If, on the other hand, the tube is formed in horseshoe shape, if it assumes that shape when relaxed, then squeezing it to reduce the difference between inside and outside circumferences will urge twisting in an opposite direction.
Thus it is that twisting is urged during pump operation whether the tubing be bent during installation or preformed to the shape it has when installed in a pump. It is true whether the tube is pinched by pressing it with the cam against a confining circular wall which encompasses the tube over its length, or whether the tube is pinched by the cam stretching the inner wall to meet the outer wall of the tube. The latter arrangement results in large forces tending to pull, then push, the ends of the tube in the direction of flow, and, partly for that reason, most peristaltic pumps include the encompassing wall. Even in such pumps, the forces that are exerted on the structures that hold the tube ends in place are sufficiently large to shorten tube life and to influence the design of the entire pump structure.
The combined effect of the twisting and pulling of the pump tubing during pumping is to cause the tube to tend to move back and forth in the direction of the axis of cam rotation. In some cases that movement is sufficient to result in rubbing of the tubing against the end walls of the cavity in which the cam and tube are housed.
The invention provides a pump and pump tube in which these several problems are minimized.